Improve nvchecker tooling

[cbix]

Currently we have to manually keep the nvchecker/old_ver.json up to date with the current package versions. That step could be eliminated by generating that json from the package sources in run-nvchecker.cron. This comes with two prerequisites:

1. package names have to exactly match the names in archlinux-proaudio.toml ✅
2. nvchecker version formats have to match the pkgver values ✅

Furthermore we should introduce individual .nvchecker.toml files in package directories since Arch Linux recently started introducing that (example). This has a few benefits:

• make package adoption easier
• more modular, no cleanup needed when removing a package